1. Field of the Invention
The present invention relates to a method for retransmission, and more particularly, to a method for dynamic packet retransmission of a radio link control (RLC) layer for a wireless communications system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipment (UE).
A radio link control (RLC) layer is responsible for data transfer of radio bearers from an upper layer, radio resource control (RRC) layer, and includes three transfer modes of Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). In the LTE system, an AM RLC entity, either in the E-UTRAN or in the UE, consists of a transmitting side and a receiving side and supports segmentation, retransmission, sequence check and other functions. The transmitting side of the AM RLC entity receives RLC service data units (SDUs) from upper layers and delivers RLC protocol data units (PDUs) to its peer AM RLC entity according to a transport block (TB) size indicated by a lower layer. The receiving side of the AM RLC entity delivers RLC SDUs to the upper layers and receives RLC PDUs from its peer AM RLC entity via the lower layer.
Due to a dynamic nature of the TB size, the LTE system supports the variable size for the RLC PDUs and RLC PDU re-segmentation. The current standard only considers re-segmentation for the retransmission when an RLC PDU to be retransmitted cannot fit the TB size indicated by the lower layer. The re-segmentation level is not limited, i.e. an RLC PDU segment can be segmented again for retransmission if it cannot fit the indicated TB size. A specific example refers to a document: 3GPP TSG-RAN WG2#59 R2-073535.
The RLC PDU segments are retransmitted if they are negatively acknowledged. For example, if a first RLC PDU segment is negatively acknowledged and the TB size for retransmission can afford the first RLC PDU segment, the first RLC PDU segment is submitted to the lower layers without re-segmentation. When two or more consecutive RLC PDU segments are negatively acknowledged, the simplest way is to retransmit them with the same format as previous transmission. This way is not efficient to utilize radio resources as illustrated by examples below.
Please refer to FIG. 1, which is a schematic diagram of RLC packets corresponding to retransmission according to the prior art. In FIG. 1, an RLC PDU consists of a header and a data field, and the data field is an RLC SDU. The RLC PDU is segmented into RLC PDU segments 1, 2 and 3 transmitted by a transmitter, which could be the UE or E-UTRAN. The header includes a D/C, an RF, a P, an SI, an E, an SN and a (E+LI)s fields. The definition of the fields are well known in the art and the detailed descriptions are omitted here.
For the first example, the RLC PDU segments 1 and 2 are negatively acknowledged, while the RLC PDU segment 3 is positively acknowledged. The transmitter has to retransmit the RLC PDU segments 1 and 2. At a transmission time interval (TTI) for retransmission, a TB size indicated from lower layers cannot afford the RLC PDU segments 1 and 2, but can afford the whole RLC PDU segment 1 plus data of the RLC PDU segment 2. In this case, only the RLC PDU segment 1 is retransmitted at the TTI, whereas the RLC PDU segment 2 has to be retransmitted at the next coming TTI. Thus, radio resources cannot be fully utilized for data transmission.
Please continue to refer to FIG. 1 for the second example. The RLC PDU segments 1, 2 and 3 are transmitted and all the RLC PDU segments 1, 2 and 3 are negatively acknowledged. The transmitter has to retransmit the RLC PDU segments 1, 2 and 3. At a TTI for retransmission, the TB size cannot afford the three RLC PDU segments but can afford the whole RLC PDU or the RLC PDU segment 1 plus the RLC PDU segment 2. In this case, only RLC PDU segments 1 and 2 are retransmitted, and the RLC PDU segment 3 has to be retransmitted at the next coming TTI. Radio resources cannot be fully utilized for data transmission as well.
Please refer to FIG. 2, which is a schematic diagram of RLC packets corresponding to retransmission according to the prior art. For the third example, the RLC PDUs 1, 2 and 3 are transmitted to the receiver, whereas the RLC PDU 4 is a new PDU that has not been transmitted to the receiver yet. The RLC PDUs 1 and 2 are negatively acknowledged, while the RLC PDU 3 is positively acknowledged. The transmitter has to retransmit the RLC PDUs 1 and 2. At a TTI for retransmission, the TB size cannot afford the RLC PDUs 1 and 2 but can afford the RLC PDU 1 plus partial data of the RLC PDU 2. In this case, only the RLC PDU 1 is retransmitted, and the RLC PDU 2 has to be retransmitted at the next coming TTI. Radio resources cannot be fully utilized for data transmission as well.
Please continue to refer to FIG. 2 for the fourth example. The RLC PDUs 1, 2 and 3 shown in FIG. 2 are transmitted. The RLC PDUs 1 and 2 include data of 500 and 100 bytes, respectively. The RLC PDUs 1 and 2 are negatively acknowledged, while the RLC PDU 3 is positively acknowledged. The transmitter has to retransmit the RLC PDUs 1 and 2. At a TTI for retransmission, the TB size is allocated with 100 bytes for data, indicated from the lower layers. In this situation, the RLC PDU 1 needs to be segmented. As well known in the art, the header of an RLC PDU segment includes two extra fields of SO and LSF, represented by a total 2 bytes. Thus, the SO and LSF fields occupy 2 bytes of 100 bytes and only 98 bytes of are used for the data of the RLC PDU 1 at the TTI. If a TB size at the next TTI is allocated with 500 bytes for data, the transmitter transmits 404 bytes, which includes 402 bytes for the remaining data of the RLC PDU 1 and 2 bytes for the SO and LSF fields, and 96 bytes for data of the RLC PDU 2. The RLC PDU 2 is segmented as well, and the remaining 4-byte data has to wait for the next transmission.
Please refer to FIG. 3, which is a schematic diagram of RLC packets corresponding to retransmission according to the fifth example of the prior art. The RLC PDUs 1 and 2 are transmitted and negatively acknowledged. The RLC PDU 1 is retransmitted with RLC PDU 1 segments 1, 2 and 3. In the transmission opportunity for the RLC PDU 1 segment 3, the TB size provides a greater capacity than the size of the RLC PDU 1 segment 3. In this situation, the RLC PDU 2 are segmented. The RLC PDU 1 segment 3 and the RLC PDU 2 segment 1 are then retransmitted together at this TTI, thereby wasting the radio resources due to one RLC PDU segment header needed for RLC PDU 2 segment 1.